1. Field of the Invention
The present invention relates to a resistance welding machine in which workpieces are tightly clamped by a pair of electrodes and are welded by welding current supplied through the electrode pair.
2. Description of the Prior Art
In a resistance welding machine including a pair of electrodes between which workpieces are tightly clamped to be welded by welding current supplied through the electrode pair, the electrode pain starts a movement to make a distance thereof short when a clamping command is entered to the welding machine. When the distance becomes equal to the thickness of the workpieces, the workpieces are clamped by the electrode pair. In the early stage of the clamping, the clamping force is not stabilized due to bounding and other factors. Thus, welding current through the electrode pair must be started just after a stabilized timing when the workpieces are properly clamped between the electrodes in a stable manner.
To this end, stabilization of the clamping force has been conventionally detected by comparison with a certain constant such as those described below.
Specifically, in a prior art, a time from a timing when the clamping command is entered to the machine until a timing when the clamping force is stabilized is previously measured by an experiment. The welding current flow is started when the elapsed time from the timing when the clamping command is entered exceeds the measured period.
It is also known that the pressure in a cylinder for tightly clamping workpieces between the electrodes is directly related to the clamping force applied on the workpieces, and that, when the workpieces are clamped between the electrodes, members supporting the electrodes develop strain which is indirectly related to the clamping force on the workpieces. Therefore, in another prior art, the value of the pressure in the cylinder or the strain developed in the supporting members at a timing when the clamping force on the workpieces is stabilized is previously measured. The welding current flow is started when the pressure in the cylinder or the strain developed in the supporting member reaches the measured value.
However, the time from when the clamping command is entered until the workpieces are stably clamped, or the cylinder pressure or the strain at a timing when workpieces are stably clamped is variable depending on various welding conditions such as thickness and material of the workpieces.
When it is desired to weld various kinds of workpieces, constants of the time, the cylinder pressure or others must be determined in respective cases so as to start welding current flow according to the respective constants. This is very cumbersome.
Even in workpieces of the same kind, the time from the clamping command entry timing until the stabilized timing may be variable due to difference in the approaching speed of the electrodes. For example, if an air supply device is connected to a plurality of cylinders (air cylinders), there will be difference in speed of driving the cylinder or cylinders between when a single cylinder is actuated and when many cylinders are actuated simultaneously. In such a case, it is very difficult to determine the stabilized timing on the basis of the time from the clamping command entry timing, and consequently to start welding current flow just after the stabilized timing.